1. Field of the Invention
Embodiments of the present invention are generally related to safety valves. More particularly, embodiments of the present invention pertain to subsurface safety valves configured to be actuated using wellbore pressure in the event of an unexpected pressure drop.
2. Description of the Related Art
Subsurface safety valves are commonly used to shut-in oil and gas wells and are typically fitted in a string of production tubing installed in a hydrocarbon producing well. The safety valves are configured to selectively seal fluid flow through the production tubing to control the flow of formation fluids upwardly should a failure or hazardous condition occur at the well surface.
Typically, subsurface safety valves are rigidly connected to the production tubing and may be installed and retrieved by conveyance means, such as tubing or wireline. During normal production, safety valves are maintained in an open position by the application of hydraulic fluid pressure transmitted to an actuating mechanism. The actuating mechanism in such embodiments may be charged by application of hydraulic pressure through hydraulic control systems. The hydraulic control systems may comprise a clean oil supplied from a surface fluid reservoir through a control line. A pump at the surface delivers regulated hydraulic fluid under pressure from the surface to the actuating mechanism through the control line. The control line resides within the annular region between the production tubing and the surrounding well casing.
In the event of a failure or hazardous condition at the well surface, fluid communication between the surface reservoir and the control line is interrupted. This, in turn, breaks the application of hydraulic pressure against the actuating mechanism. The actuating mechanism recedes within the valve, allowing a flapper to quickly and forcefully close against a corresponding annular seat—resulting in shutoff of the flow of production fluid. In many cases, the flapper can be reopened (and production flow resumed) by restoring the hydraulic fluid pressure to the actuating mechanism of the safety valve via the control lines.
For safety reasons, most surface controlled subsurface safety valves (such as the ones described above) are “normally closed” valves, i.e., the valves are in the closed position when the hydraulic pressure in the control lines is not present. The hydraulic pressure typically works against a powerful spring and/or gas charge acting through a piston. In many commercially available valve systems, the power spring is overcome by hydraulic pressure acting against the piston, producing axial movement of the piston. The piston, in turn, acts against an elongated “flow tube.” In this manner, the actuating mechanism is a hydraulically actuated and axially movable piston that acts against the flow tube to move it downward within the tubing and across the flapper.
Safety valves employing control lines, as described above, have been implemented successfully for standard depth wells with reservoir pressures that are less than 15,000 psi. However, wells are being drilled deeper, and the operating pressures are increasing correspondingly. For instance, formation pressures within wells developed in some new reservoirs are approaching 30,000 psi. In such downhole environments, conventional safety valves utilizing control lines are not operable because of the effects of hydrostatic pressure on the hydraulic fluid within the control line. In other words, high-pressure wells have exceeded the capability of many existing control systems, especially hydraulic control systems which rely on control lines, which are susceptible to reliability problems.
Therefore, a need exists for a subsurface safety valve that is suitable for use in high pressure environments. There is a further need for a subsurface safety valve that does not rely on a control system that requires the use of control lines conveying hydraulic fluid to an actuating mechanism. There is yet a further need for the ability to reopen the safety valve remotely from the surface of the well.